Hazards induced in infrastructure projects by unrevealed geological features

This work presents some obvious relations between the hydrogeological structure, the presence of “large swell/shrink soils”, their mineralogical composition and geomechanical properties, and the ubiquitous landslides on Peri Carpathians Hills. Large infrastructure projects offer the opportunities to put into evidence the swelling properties of Upper Pliocene – Lower Pleistocene deposits, which support the Holocene alluvial deposits of various rivers. Swelling and shrinking properties refer to significant positive or negative variations of volumes due to the absorption or desiccation of water in fine soils under natural or anthropic moisture regimes. These physical phenomena are spread worldwide and have important engineering consequences with an associated cost of damages of several billion annually in all climate areas. In spite of the fact that these geotechnical properties have been studied for more than eight decades, the particularities of these peculiar relations between water, mineral composition, and geomechanical behaviour are still unrevealed entirely. In Romania, swell/shrink soils are reported in all regions at different depths, but they are not taken into serious consideration in the literature and are rarely related to geotechnical engineering accidents such as slope slides or road failures. Analyzed samples allow us to define some basic correlations between plasticity index, colloidal fraction, dry density, swelling pressures or free swelling, and mineralogical composition.

INTRODUCTION

Worldwide occurrence of expansive soils

Often referred to as expansive soils in international literature, soils with large swelling and shrinking potential, are defined as natural materials that exhibit volume variations related to variations of moisture.

After more than eight decades of international studies, the unpredictability of the behaviour of expansive soils derived from the tight dependence between mineralogical fabric and composition and the pattern of chemical watering or desiccation processes remains very high and, thus, still unrevealed completely.

Expansive soils are implicitly pointed out associated with large damage to onshore or offshore, over or underground infrastructure projects in more than 60 countries, on all continents except the Antarctic one, often after long periods of drought, heavy rains, freezing or unfreezing.

Their broadening is usually incompletely presented in geologic or pedologic maps, only for the superficial parts of the ground associated with highly overconsolidated claystone and clay shale (NELSON et al., 2015).

Despite worldwide efforts to depict, understand and predict these peculiarities, annual damages related to geomechanical processes produced by variations of water content in expansive soils are reported all over the world. For instance, in the UK – £400 million, (DRISCOLL and CRILLY, 2000), or in the USA – $15 billion (NELSON and MILLER, 1992). In the UK, or in China where expansive soil covers more than one hundred thousand square kilometres, expansive soil behaviour is stated as one of the most dangerous geological hazards that affect large and diverse engineering projects (JONES and JEFFERSON, 2012, GASPAR et al., 2022, XIAO et al., 2018).

In Romania, expansive soils are spread over the country, in all geological units, at the surface of the ground or in deep depths. The national normative NP-126-2010, based on punctual reports, presents a sketch with rogue surfaces of soils with moderate or high swell/shrink potential, situated at the top of the terrains.

The mineralogical nature and identification tests of expansive soils

All swelling processes are related to clay minerals, known as hydrous phyllosilicate having silica, alumina and water with variable content of inorganic ions like Mg²⁺, Na⁺, and Ca²⁺  which are basically divided into several classes depending on the ratio of tetrahedral silica vs. octahedral alumina structure (KUMARI and MOHAN, 2021). There are four major groups of clay minerals that are present in soils. Among them, the layer group is the most important one, which comprises: type 1:1 (kaolinite, halloysite, serpentine); type 2:1 (smectite, vermiculite, illite, mica) and type 2:1:1 (chlorite).

The largest swelling potentials are related to the presence of minerals from the smectite family, which contains saponites, montmorillonite and bentonite, and absorb the largest quantities of water between clay sheets (KUMARI and MOHAN, 2021, MITCHELL and SOGA, 2005).

Physico-chemical characteristics of minerals which dictate the amplitude of swelling potential are, in order of impact: average specific surface area, particle thickness, interlayer space and cation exchange capacity. Table 1 presents these characteristics specific to some usually encountered clay minerals.

Table 1: Physico-chemical characteristics of minerals (KUMARI and MOHAN, 2021, YONG et al., 2012)

Every particular soil layer is a mixture of clay minerals with other particles of inert mineral type (quartz, feldspar etc), a fact that triggers a unique swelling and shrinking behaviour. It is considered that clays which are geological materials of great mineralogical variety may exhibit swelling properties if containing mineral particles more than 50% with less than 2-micron size. Clays containing montmorillonite show an almost reversible swelling and shrinking on rewetting and redrying, whereas clays containing kaolinite or illite show an initial large volume decrease on drying with only limited swelling on rewetting (YONG et al., 2012).

Various procedures for the identification of expansive soils may be established following the international literature, national codes, or professional guides, which are based on a large variety of parameters, indices, or techniques, however, none of those being comprehensive and worldwide accepted. Thus, in countries with a predominantly arid climate, the geotechnical methodology for expansive soil characterization is based on shrinkage tests, while in temperate climate swelling tests prevails.

JONES and JEFFERSON, (2012) differentiate the following laboratory testing classes: (i) swelling tests, which may be divided into swelling strain and swelling pressure tests; (ii) index tests based on basic parameters such as liquid limit, plastic limit, plasticity index (usual or modified); (iii) oedometer-based methods, which may be free swell tests or constant volume tests; (iv) suction-based tests which use soil-water characteristic curves; (v) mineralogical tests.

NELSON et al., (2015), define three classes of identification methods: (i) based on physical properties – plasticity, free swell test, potential volume change, expansion index, linear extensibility and standard absorption of moisture content; (ii) based on mineralogical composition – X-ray diffraction, differential thermal analysis and electron microscopy; (iii) based on chemical analyses – cation exchange capacity, specific surface area and total potassium content.

Based on the above-defined tests, various schemes of characterization and classification have been developed worldwide, which may be generally categorized into four groups (JONES and JEFFERSON, 2012): free swell, heave potential, degree of expansiveness and shrinkage potential.

Romanian normative NP 126:2010 proposes a scheme of characterization based on a particular collection of parameters: colloidal fraction (A₂μ), plasticity index (I_p), activity index (I_A), plasticity criterion (C_p), free swelling (U_L), shrinkage limit (w_s), volumetric shrinkage (C_v), maximum wetting heat (q_max), moisture content at 15 bar suction (w₁₅) and swelling pressure (p_u).

Micro and macroscale “symptoms” of expansive soils

In the microscale register, swelling and shrinkage mechanisms of expansive soils may be explained by two major theories: crystal swelling of clay mineralogy, in which the water molecules get combined with the cations associated with mineral flakes to form hydrated ones, and a diffuse double layer of colloidal chemistry, in which around negatively charged clay minerals hydrated ions and polar water molecules are firmly adsorbed and form a fixed layer and a thick diffusion layer, thus widening the spacing between mineral particles (ZHOU et al., 2019). During the swelling process, the inter-clay bonds are weakened or broken, the layer structure of clay minerals is deformed, and in the macroscopic field, the strength of the soil decrease.

A well-accepted knowledge is that the shrink-swell potential is dictated by its initial water content, void ratio and vertical stresses, as well as the type and amount of clay minerals in the soil (BELL and CULSHAW, 2001). Numerous researchers underlined a specific pattern of behaviour since BISHOP (BISHOP et al., 1975), up to ALONSO (ALONSO et al., 1990), and claim the collapse of expansive soil after a single swell/shrink cycle. Many others sustained the decrease of shear strength from 3-5.5 times (SHERIF et al., 1984), up to 70% to 90% in a full swelling circumstance (AL-MHAIDIB and AL-SHAMRANI, 2006), mostly by the reduction of effective cohesion, sometimes very close or equal zero with dramatic consequences in geomechanical behaviour (LAN et al., 2022).

As swelling, the shrinkage also is not a reversible process because the cracks created may not always completely close up after rewetting and allow water to penetrate deeper in the stratum and sometimes fill them with sediments (KHADEMI and BUDIMAN, 2016).

Most often, the swelling/shrinking processes are reported in the near-surface zone and are related to seasonal variation of underground water content (JONES and JEFFERSON, 2012), which may extend on variable depths from 1.0 m to 5-6 m (XIAO et al., 2018), according to climate particularities, drying/rehydration regime and mostly to variation of temperature. Nonetheless, similar processes may be present at lower depths, by the water flow through more permeable layers (in contact with expansive soils) charged from distant front-loading sections (NELSON et al, 2003).

Usually, swelling of expansive soils is considered only in the vertical direction, but it has been proved that lateral swelling pressure develops additional stress to the lateral earth pressure on retaining walls which may increase at the bottom of the wall equal to 1.3-4 times the overburden (ABDULAHI and VAHEDIFARD, 2020), and reduce the bearing capacity of piles (DA SILVA BURKE et al., 2022, NELSON et al., 2015).

SWELLING GEOHAZARD CONDITIONS OF MAJOR PROJECT IN ROMANIA

Geologic and hydrogeologic regional frame

The expansive soils we refer to belong to a large structure attributed to Upper Pliocene – Lower Pleistocene named “Candesti Layers”. This huge formation is part of a sedimentary complex named Dacic Basin which is disposed on the external side of the Carpathian Chain (Oriental, Curvature, and Meridional), over the Getic Depression and Moesian Platform (JIPA, 2006). The specific sedimentary structure – Candesti Layers – has a variable spatial shape, with widths starting from 6-8 km in the Curvature area, to more than 80 km in the western Meridional Carpathians and thickness which may exceed 250 m (PALCU et al., 2008). From a hydrogeological perspective, Candesti Layers is defined as a large multilayer-aquifer structure characterized by hydraulic conductivities up to 100 m/day and transmissivity less than 1,000 m²/day, with a thin feeding front in the North, end at the contact with the Carpathian Chain and main underground flowing directions from Nord to South. This regional hydrogeologic configuration is in hydraulic contact with deeper regional aquifers and may develop water pressures on the cover strata up to 40 Bar. The whole package of multilayer-aquifer structures (from Jurassic to Upper Pleistocene) function under a pressure regime which becomes patchy artesian and is defined as an “Artesian Dacic Basin” (Fig. 1), (PALCU et al., 2008). The swelling geohazard derives from the rhythmic sedimentation regime which is materialized in alternant sequential layers separated in two main terms: the coarse one (gravels and sands) and the fine one (expansive clays and fine sand), disposed in the specific geologic structure which is presented in Fig. 2.

Figure 1: Extent of the Artesian Dacic Basin (PALCU et al., 2008)

Figure 2: Hydrogeological section between V. Argesului and V. Mosoaia (FERU et al., 1980)

 Characterization of expansive soils belonging to Candesti Layers

The expansive properties of Candesti Layers have been evaluated based on some of the parameters stipulated by the Romanian Normative for Expansive Soils (NP 126:2010). The main parameter considered in this paper was the swelling pressure, p_u (kPa) which was measured according to STAS 8942/1-89. The results are divided into three classes of pressure which are graphically exposed according to depth in Fig. 3, which allow the observation that the III^th class containing the greatest swelling pressures is developed between 10 and 30 m depth.

For all the three classes defined above, samples have been identified and classified according to European and Romanian regulations (SR EN ISO 14688-2). As it is depicted in Table 2, the first class of swelling pressures (p_u<200 kPa) is constituted on all four types of fine soils in balanced percent; instead, for the latter classes, the clay prevails on account of the other types but especially of sandy silty clay (sasi Cl) which completely disappear in the most powerful class of swelling pressure (p_u>400 kPa).

Figure 3: Variation of swelling pressure of Candesti Layers in depth

Table 2: Granulometric characterisation of Candesti Layers samples

The next step was to define the activity of soil samples based on the activity index I_A=I_P/A_2μ (NP 126:2010). Based on this calculation, in spite of numerical values (which are, for all swelling pressure classes, close to 1), the positions in the activity fields lead to the qualitative observation that when p_u>200 kPa all the samples may be regarded as very active in contact with water. As part of the specific characterization of Candesti Layers, several mineralogical analyses have been executed, which specified that the swelling minerals prevailed in all samples (classes II and III) as is presented in Table 3.

Table 3: Mineralogic characterisation of Candesti Layers samples

Calculation of the depth of the potential heave

Considering the hydrogeological frame exposed above, which provides conclusive evidence that at a regional scale the Candesti Layers aquifer submits the expansive soils (which are part of it) to constant ascendent vertical drainage at up to 40 Bar pressures, we appreciate that the risk of moisture variation remains rise on the whole thickness of the expansive soils. In consequence, we evaluate the depth of the potential heave as the maximum depth at which the overburden vertical stress equals or exceeds the swelling pressure of the soil. Figure 4 presents the variation in depth of the negative difference between the swelling pressure of samples and the lithostatic pressure at that level.

Figure 4: Extension in depth of the potential heave of Candesti Layers

Figure 5: Chart of the extension in depth of the potential heave of Candesti Layers

 One can observe that these values pass over 100 kPa just below the terrain surface and achieve a maximum of 500 kPa at 25 m depth. These values extend the depth of the potential heave which may be considered as the maximum depth of the active zone, to almost 28 m. These calculations may be considered as a qualitative first step in geotechnical calculations, which points to the area where the swelling behaviour of soils may exceed the equilibrium of the geological structure and produce a variety of effects such as lumps or even landslides in areas with sloping terrain. Quantitative estimations of free-field heave and ultimate heave parameters, useful for a proper design of any construction, may be obtained based on consolidation-swell tests and constant volume tests executed for all individual strata above the depth of the potential heave, through a calculation procedure mirrored to settlement calculation (NELSON et al., 2001, 2003, 2015). For large infrastructure projects developed over areas where expansive soils are present not only in the upper part of the terrain, this procedure may be used in the preliminary stages of investigations to reveal the extension of these types of soils and to indicate locations and depths where sampling and testing must be densified and adapted to the requirements imposed by the presence of “large swell/shrink soils”. In this regard, Figure 5 presents a chart of the variation of swelling pressure in-depth, marked with the levels where overburden vertical stress is smaller than it, which defines the deeper limit of the active zone of dangerous large swell deformations.

CONCLUSIONS

The paper presents some of the results of geotechnical investigations executed on a large geological formation, Candesti Layers, which is present at the surface or covered at small depths on the external side of the Carpathian Chain (Oriental, Curvature, and Meridional). By considering the whole investigated structure from a hydrogeological point of view, we integrated the sedimentary fabric and pressiometric regime of this large multistate-aquifer with the basic geotechnical properties which define expansive or “large swell/shrink soils”.

In this manner, we achieved some helpful conclusions which may be useful for further geotechnical studies executed in the area of occurrence of this particular formation, especially for complex infrastructures purposes:

• Candesti Layers is a geological formation characterized by the rhythmic alternances of coarse and fine soils, which is in hydraulic contact with deeper regional aquifers and may develop water pressures on the cover strata up to 40 Bar (FERU et al., 1980, PALCU et al., 2008). In this context, the expansive layers may be moist at the surface of the terrain by infiltrations and wetted or even submerged in deeper positions by ascensional drainage;
• for this particular formation, specific values of swelling pressures have been revealed which vary from 10 kPa up to 980 kPa;
• the magnitudes of the swelling pressures are obviously and directly related to granulometric compositions, the more significant pressures (p_u>400 kPa) being specific for clays and sandy clays; correlations between other geotechnical parameters usually measured for characterization of expansive soils were found wick, facts which allow the presumption that this key parameter (p_u) must be influenced by other properties, perhaps the mineralogic composition of clay aggregates;
• the evaluation of the depth of the potential heave, as the maximum depth at which the overburden vertical stress equals or exceeds the swelling pressure (NELSON et al., 2001, 2003, 2015), offers quantitative spatial indications regarding the active zone in which the strata are affected by swelling/shrinking processes;
• detailed and specific calculations of the free-field heave may be obtained based on consolidation-swell tests and constant volume tests executed for all individual strata above the depth of the potential heave, which suppose a denser sampling and more specific laboratory testing;
• as final conclusion, we estimate that the large uncertainty and large variably related to mechanical behaviour of expansive soils must be attenuated at least for large infrastructure projects situated on this type of soil, by:
• increasing the depth of investigation in order to define correctly and completely the geologic-geotechnical model of foundation terrain;
• densification of information by increasing the number of samples and specific geotechnical laboratory tests at such a level to allow a proper calculation of the depth of the potential heave and of the free-field heave;
• execution of “forensic hydrogeological studies” which must find and define all the access ways of underground water flow in the structure, in correlation with the regional pressiometric regime.

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Authors: 

Mihaela STANCIUCU – University of Bucharest, Department of Engineering Geology

Adrian Mihai DIACONU – Geotesting CI, Bucharest

[Proceedings of the 17th Danube European Conference on Geotechnical Engineering (17DECGE), June 7-9, 2023, Bucharest, Romania – https://17decge.ro/]

…citeste articolul integral in Revista Constructiilor nr. 206 – septembrie 2023, pag. 65-69